Author: Walter Faxon
Date: 21:49:11 12/05/02
Go up one level in this thread
On December 02, 2002 at 20:27:39, Gerd Isenberg wrote:
>Ahh.. i see,
>
>a special kind of popcount for consecutive trailing "ones".
>I love this interactive programming lessions.
>
>Gerd
>
>Btw.: Impressive assembler listing:
>
>00401003 8B 4C 24 0C mov ecx,dword ptr [esp+0Ch]
>00401007 8B 31 mov esi,dword ptr [ecx]
>00401009 8B 79 04 mov edi,dword ptr [ecx+4]
>0040100C 8B D6 mov edx,esi
>0040100E 8B C7 mov eax,edi
>00401010 83 C2 FF add edx,0FFFFFFFFh
>00401013 83 D0 FF adc eax,0FFFFFFFFh
>00401016 23 F2 and esi,edx
>00401018 23 F8 and edi,eax
>0040101A 89 31 mov dword ptr [ecx],esi
>0040101C 89 79 04 mov dword ptr [ecx+4],edi
>0040101F 8B CF mov ecx,edi
>00401021 33 D6 xor edx,esi
>00401023 33 C1 xor eax,ecx
>00401025 33 C2 xor eax,edx
>00401027 5F pop edi
>00401028 35 81 FC C5 01 xor eax,1C5FC81h
>0040102D 5E pop esi
>0040102E 8B D0 mov edx,eax
>00401030 C1 EA 10 shr edx,10h
>00401033 03 C2 add eax,edx
>00401035 33 D2 xor edx,edx
>00401037 8B C8 mov ecx,eax
>00401039 C1 E9 08 shr ecx,8
>0040103C 2A C1 sub al,cl
>0040103E 25 FF 00 00 00 and eax,0FFh
>00401043 8A 90 D5 62 40 00 mov dl,byte ptr [eax+4062D5h]
>00401049 8B C2 mov eax,edx
>0040104B C3 ret
Hi, Gerd.
I have noticed that most compilers aren't too smart where it comes to bitwise
dataflow analysis. After the "add eax, edx" instruction above, better (well,
shorter!) code would be:
sub al, ah
and eax, 0FFh
mov al, byte ptr [eax+4062D5h]
ret
Perhaps if we added a little "hint" in the last 2 lines of source:
inline // inline declaration may differ by compiler
u8 LSB_64( u64* bb )
{
u64 t64;
u32 t32;
u8 t8;
t64 = *bb - 1;
*bb &= t64; // omit this line to retain current LSB
t64 ^= *bb;
t32 = (u32)t64 ^ (u32)(t64 >> 32);
t32 ^= LSB_64_magic;
t32 += t32 >> 16;
t8 = (u8)t32 - (u8)(t32 >> 8);
return LSB_64_table [LSB_64_adj + t8];
}
Of course, this really only helps because we already know that the x86 class can
independently address both the first and 2nd bytes of a register. And it's
wishful thinking that much better asm code will actually be produced! I'd be
interested to see.
If/when you run your test suite again, please try this version. Thanks.
-- Walter
This page took 0.01 seconds to execute
Last modified: Thu, 15 Apr 21 08:11:13 -0700
Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.